The effect of acid-base status on intracellular pH and high energy phosphate content during hypothermia is unknown in warm- blooded animals. What is known is that changes in intracellular pH have widespread implications for metabolic functions of the cell. The variation of intracellular pH with temperature has been well characterized by comparative physiologists for hibernators and ectothermic animals. However, no data has been accumulated on the changes in intracellular pH of the heart and brain in warm blooded animals. This study has important implications for the conduct of cardiac surgery and hypothermic cardiopulmonary bypass in man. The hypothesis to be tested is that by maintaining pH parallel to the neutral pH of water (that is, increasing pH during hypothermia), the intracellular pH of the heart and brain will also follow the neutral pH of water and that this will be beneficial for the cellular energy state. An additional hypothesis is that a correlation can be found between two methods of pH measurement (NMR and the NIH pH probe). The experimental model consists of 8 to 10 week old sheep. This model was chosen because sheep past this age have very little 2,3- DPG in the blood. The Nuclear Magnetic Resonance (NMR frequency of inorganic phosphate reflects intracellular pH. Because 2,3-DPG resonates at a frequency near inorganic phosphate, it is necessary to have a model free of 2,3-DPG with a radiofrequency coil on either the heart or brain. The animal will be cooled using surface cooling in the bore of the magnet. Nuclear magnetic resonance spectra will be used to assess the changes in intracellular pH and in phosphorylation potential during hypothermia as a function of blood pH for the heart and brain.